1. Field of the Invention
The present invention relates to munitions, and it particularly relates to an orientation referencing system designed to measure a munitions angular orientation and position (distance from a ground point) with absolute onboard referencing. This orientation referencing system offers advantages to the guidance and control of smart munitions. Other advantages of the orientation referencing system implementation include: (1) the capability of a smart munition with such a sensing system to be capable of determining its position and orientation while in flight with respect to a point on the ground, (2) the capability of a munition to have autonomous orientation as a single munition (no requirements to triangulate with other munitions) to provide the capability of one hit, one kill, and (3) the sensing system will be minimally intrusive and consume relatively very low power.
2. Background of the Invention
Future combat systems will require a significant reduction in weight and a significant increase in performance for both platforms and munitions. Significant gains have been achieved in the areas of Micro-Electro-Mechanical (MEMs) technologies which, in turn, have made possible a significant reduction in the size and weight of accelerometer mechanisms and substantial performance enhancement of these devices under extreme operating conditions.
To take advantage of these advances and to meet the requirements of the U.S. Army""s future needs in the areas of precision-guided direct-and indirect-fire munitions, it is important to develop sensors with unique characteristics that can be integrated reliably and economically into small-and medium-caliber munitions as well as long-range munitions. In particular, it is desirable to embed electromagnetic sensors in the munitions to enhance the guidance and control of munitions in flight and improve their accuracy. These sensors will provide real-time information on the orientation, rotation and position of munitions as well as define the orientation vectors of the center of mass. Such real-time information, used alone or in conjunction with gyroscopes and inertial measurement units (IMUs) will enable the deployment of a new generation of affordable munitions characterized by highly-steerable guidance and navigation control systems that will guide them to their targets with unprecedented accuracy, delivering heretofore unimaginable lethality with a minimal expenditure of ammunition.
Radar-based guidance of munitions is currently based upon the use of radio frequency (RF) antennas printed or placed on the surface of munitions to reflect RF energy emanating from a ground-based radar system. The reflected energy is then used to track the munition or the stream of bullets on the way to the target. In this scenario, the targeting radar illuminates both the munition and the target. Energy reflected from the munition and target provides tracking information. The radar measures the time difference between the return signals from the munitions in flight and from the target to determine munition angle information and to determine of the munitionxe2x80x3s rotation and up-down reference.
RF antennas receive and radiate energy as a function of their size and geometric shape. Typically, a tracking radar is in the Ku band with a nominal frequency range of 12.4 GHz to 18 GHz and a freespace wavelength of 1.67cm-2.4 cm. This largely defines the size of the antennas that can be used. A narrow beam width of the reflected signal is important in achieving the required measurement of the difference between the pulse received from the illuminating radar and the target return and an accurate measurement of the roll angle for the munition.
Maximizing the magnitude of the return from the munition is important to distinguish the munition from xe2x80x9cclutter.xe2x80x9d However, it is quite challenging to shape the antenna radiation pattern and its beam width for a munition. In particular, methods that rely of measuring the reflected radar energy currently require that the sensors (RF antennas) be placed on the surface of the munition. The added requirement that the antennas be gun-hardened and survivable during the extreme launch environment characteristic of small and medium caliber munitions greatly limits the effectiveness of the current generation of antennas.
Corrections to a munitionxe2x80x3s flight path are currently possible but only if the munitions are equipped with an additional suite of internal sensors such as Inertia Measurement Unit (IMUxe2x80x3s), accelerometers, and gyroscopes. These sensors are relatively complex and inaccurate for the intended purpose herein. In addition, these sensors require inputs from a Global Positioning System (GPS) to define the munitionxe2x80x3s orientation vectors.
The inherent problems associated with these components are manifold. The position of the munition is determined by a double integration of the acceleration signals sensed by onboard accelerometers. Integration errors and accelerometer drift interject various inaccuracies that become intolerable when these sensed and processed signals are used to determine the error signal for the control system of the munition.
Error signals should not depend on the signals being sensed or processed outside the munition while in flight. The dependency on onboard accelerometers, IMUs, gyroscopes, or other inertial sensing mechanisms that require other external calibration and correction signals, make the feedback loop so large that the resulting onboard error signal does not accurately represent the midcourse correction needed to strike the intended target. Moreover, reliance on GPS as part of the control system is problematic as GPS can be electronically jammed.
Hence, the current suite of electromagnetic sensors and even electromagnetic sensors augmented by accelerometers, IMUs, gyroscopes, and GPS, are incapable of meeting the increasingly stricter requirements for precision delivery of munitions. A new generation of sensors is thus required to meet this need.
These new sensors should be capable of providing onboard information about the angular orientation of the munition, i.e., its pitch, yaw, and roll angles, as well as its absolute position relative to a ground command station or the target (moving or stationary).
It is also desirable to have the capability to configure the sensory systems to provide onboard information about the position and orientation of the munition relative to an incoming target so that they can be used as a homing sensor. To achieve this goal, the sensors should require minimal or no computational and signal processing capability at the ground (command) station. By minimizing the role of the ground or command station in the guidance and control loop, the related sources of error and system and operational complexity that are otherwise introduced into the overall system can generally be minimized or eliminated.
In particular, the feedback loop needs to be kept as small as possible, thus requiring an autonomous onboard referencing orientation and position sensor system. In addition, the development of affordable guidance and control technologies is dependent on the development of accurate and reliable position and orientation sensors that are inexpensive to produce as well as easy to integrate into various munitions.
Currently, there is no means of meeting these requirements. Thus, there is a great and still unfulfilled need for a system of efficaciously and economically embedding guidance and control components in gun-fired munitions.
The development of an autonomous onboard absolute position and orientation referencing system (also referred to herein as xe2x80x9cthe referencing systemxe2x80x9d) of the present invention fills this void by providing a means of efficaciously and economically embedding guidance and control components into the fins of supersonic, highly maneuverable small, medium-caliber and long range munitions. Embedded resonant cavities, which are an integral part of the on-board guidance and control system, receive, rather than simply reflect, electromagnetic energy from a ground-based radar source.
The magnitude and phase information received by the integral antennas is then used to determine the munition orientation, using a minimum of onboard electronics. Embedded sensors will provide continuous, onboard information about the angular orientation of the munition, i.e., its pitch, yaw, and roll angles, as well as its absolute position relative to a ground station.
Further, vector information will be sufficient for determining munition center-of-mass vectors and velocity rates during flight from close range to extended ranges. The disclosed sensors may be used alone or in conjunction with accelerometers, e.g., MEMS, accelerometers, rate gyroscopes, or IMUs, to correct for drift and other accumulated errors during flight in order to achieve downrange delivery accuracy against stationary and moving targets.
Advantageously, the same sensors that are used to receive radio frequency information can also transmit information, thus establishing full duplex communication with the ground station, enabling guidance and control information to be exchanged and accuracy to be enhanced. In particular, the waveguide cavities are in resonance when the polarized electric field is aligned with the preferred shorter dimension of the embedded aperture.
As the waveguide cavity rotates by 90 degrees, the waveguide is no longer in resonance. However, radar energy emitted from the illuminator is reflected back to the ground station in both cases. The characteristics of the reflected energy in both cases (in and out of resonance) contain information about the position and angular orientation of the munition. In fact, when the cavity is totally out of resonance, the reflected radar signal on the ground will contain information that defines the munitionxe2x80x3s distance and orientation.
This information can be used to validate the orientation and position information sensed by the munition when the cavity is in resonance. Ultimately, this means that the embedded waveguide structures facilitates the implementation of an onboard sensor system with an onboard reference that allows the ground station to validate the performance of the munition and its onboard guidance and control sensors such as IMUs, gyroscopes, accelerometers, or like sensors.
In addition to its use as a means of validation, radar energy can be used to modulate information. Additional radio frequency links can be embedded to communicate between the ground and the munition.
A preferred implementation of the present invention relies on embedding in the fins of munitions a multiplicity of resonant apertures with rectangular cross-section. The resonant apertures are, in turn, equipped with an internal RF antenna placed at a strategic point within the cavity.
The resonant apertures equipped with internal RF antennas, which may be referred to generally as slot waveguide antennas, provide onboard orientation information based on the magnitude and phase of incoming electromagnetic energy provided by tracking radar. The resonant apertures may take the form of mechanically tuned waveguides, such as sectoral horn waveguides, that are machined into the fins or planar devices that rely on engineered electric permittivity profiles. By extension, resonant waveguide cavities can also be embedded in the body of the projectile to provide various functionality such as onboard information about the position and orientation of the munition relative to an incoming target so that they can be used as a homing sensor.
The invention of the present invention present numerous features, and in particular, the referencing system can be advantageously and economically integrated into the munition to receive radio frequency signals onboard the munition while it is in flight. Signals emanating from the ground station, which allow the munition to achieve heretofore unattainable levels of accuracy, need only be transmitted for very short periods of time and with a low duty cycle while providing the necessary information for a resonant cavity to operate. These resonant cavities may be thought of as special onboard RF antennas.
The principle of operation of the referencing system relies on the resonant receiving characteristics of mechanically tuned waveguide cavities designed into the structure to receive radio frequency signals. In an exemplary embodiment, the cavities are machined onto the fins or the body of a munition. The design of such waveguides can further take advantage of planar devices built from materials with specific dielectric profiles that represent alternatives for engineered devices out of homogeneous substrate dielectric materials.
The waveguide operates as a resonance type antenna when the propagated electric field aligns with the shorter side of the cavity. The waveguide is sensitive to polarization in a specific direction, which polarization is emitted from the illuminator on the ground. The illuminator sends a signal which is absorbed by the waveguide when it is in a first position (absorptive mode). When the waveguide is rotate 90 degrees, it reflects the signal (cross-polarized mode).
While any number of RF antenna types are available to implement the referencing system, a preferred antenna is a sectoral (or another type) waveguide antenna, because its geometry is inherently compatible with the fin geometry for a munition. Furthermore, the waveguide is inherently capable of withstanding the large accelerations associated with medium and small munitions. The waveguide cavities can be embedded in other parts of the projectile.
The asymmetry of the waveguide design is utilized to provide information related to the relative orientation of the waveguide with respect to the polarized illumination beam. Specifically, the resonant cavity design allows the referencing system to take advantage of low attenuation of the illumination beam in one axis and high attenuation in the other. In particular, these sensors are sensitive to the input polarization in the H-plane and will attenuate the incoming energy in the plane orthogonal to the H-plane (i.e., the E-plane).
In particular, the referencing system takes advantage of the directional sensitivity of rectangular apertures to polarized microwave illumination emanation from a ground-based source (tracking radar) which is transmitting a linearly polarized beam with a known orientation. The polarization thus establishes a known plane of reference as set by a ground source.
The received microwave energy provides magnitude and phase information from which directional/orientation data may be derived. Additionally, when the embedded waveguides are placed a known distance apart along the length of the projectile they can determine the time delay for the signal from the ground illuminator to reach the munition at each waveguide. The precise time delay relates to the velocity and distance of the munition from the illuminator.
Thus, munition with embedded waveguides can measure orientation and distance of the munition from the ground illuminator. This information available onboard the munition in flight will provides an autonomous onboard absolute position and orientation referencing system and results in an absolute onboard position and orientation referencing system for munitions in flight. In all cases, the munition recognizes its position and orientation relative to the launch platform.